Conventional ceramic materials having large piezoelectric constants, such as lead zirconate titanate, (PZT) are used for piezoelectric components such as piezoelectric actuators, piezoelectric sounding bodies, and piezoelectric sensors.
In recent years, many attempts have been made to develop lead-free piezoelectric ceramic materials, or which contain substantially no lead, because of concerns about the influence of lead on the environment.
Niobate compounds are examples of the lead-free piezoelectric ceramic materials, particularly alkali niobate-based composite oxides, which have high Curie points and relatively large electromechanical coefficients and therefore are expected to be possible ceramic materials for piezoelectric components.
Patent Document 1 discloses a piezoelectric ceramic composition which is one of the alkali metal niobate-based piezoelectric ceramic compositions. The piezoelectric ceramic composition has an open porosity of 0.4% or less by volume and contains a metal element and a principal component represented by the formula I {Lix(K1-yNay)1-x} (Nb1-z-wTazSbw)O3, wherein 0≦x≦0.2, 0≦y≦1, 0<Z≦0.4, and 0<w≦0.2. The metal element is at least one selected from the group consisting of Ag, Al, Au, B, Ba, Bi, Ca, Ce, Co, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Ho, In, Ir, La, Lu, Mg, Mn, Nd, Ni, Pd, Pr, Pt, Rb, Re, Ru, Sc, Si, Sm, Sn, Sr, Tb, Ti, Tm, V, Y, Yb, Zn, and Zr. The ratio of the metal element to the principal component is 0.0005:1 to 0.15:1 on a molar basis.
The alkali metal niobate-based piezoelectric ceramic compositions are usually resistant to sintering and therefore have a low apparent density and have pores in their surface and/or internal portions after being fired.
As disclosed in Patent Document 1, the piezoelectric ceramic composition contains 0.0005 to 0.15 mole of the metal element per mole of the principal component, which is represented by the above formula. The metal element serves as a sintering aid for increasing density. This allows the piezoelectric ceramic composition to have improved sintering resistance, high apparent density, and a smaller number of pores.
Patent Document 2, as well as Patent Document 1, discloses a piezoelectric ceramic composition containing a metal element and a principal component represented by the formula {Lix(K1-yNay)1-x}(Nb1-z-wTazSbw)O3, wherein 0≦x≦0.2, 0≦y≦1, 0<Z≦0.4, and 0<w≦0.2. The metal element is at least one selected from the group consisting of Mg, Ca, Sr, and Ba. The ratio of the metal element to the principal component is 0.0001:1 to 0.10:1 on a molar basis.
As disclosed in Patent Document 2, the piezoelectric ceramic composition contains 0.0001 to 0.10 mole of the metal element per mole of the principal component, which is represented by the above formula. This allows the piezoelectric ceramic composition to have improved properties such as a high piezoelectric constant and high mechanical strength.
The alkali metal niobate-based piezoelectric ceramic compositions are inferior in piezoelectric properties to PZT-based piezoelectric ceramic compositions.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-244300
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-244301